Efforts are being directed towards the prevention or control of graft-versus-host disease in human allogeneic bone marrow transplantation. Such graft-versus-host disease is mediated by alloreactive T cells in the inoculated marrow. With respect to the prevention of graft-versus-host disease then, reagents and techniques have been developed to remove these T cells from the marrow inoculum and to measure the success of that depletion. To this end, several murine monoclonal antibodies specific for antigens expressed on human T cells have been developed, three of which are cytotoxic. These antibodies have been utilized, in conjunction with other depletion techniques, for complement-mediated lysis of T cells in marrow. By a clonogenic assay now available, residual T cells in marrow following such a depletion are at a level of less than 0.1% of the total cell population. With respect to the control of alloreactive T cells mediating graft-versus-host disease, studies on the origin or generation of such alloreactivity have been undertaken in murine radiation bone marrow chimeras. It has been shown that the generation is influenced by a unique interaction of T cell genotype and the T cell maturation environment. The control by specific suppressor cells of alloreactive T cells resulting from the generation of this repertoire has been further studied in human alloreactive and putative suppressor T cell clones.